Dating rocks with radioactivity

Dating rocks with radioactivity

Exhibition Text

Several radioactive elements are useful for dating, depending on how rapidly they decay. For old rocks, a radioactive element with a very long half-life is needed. One such element is samarium, which is present in minuscule amounts in most rocks and minerals. Radio-active samarium transforms to neodymium with a half-life of 106 billion years. These elements have been used to determine the age of the Stillwater Complex, a body that solidified from molten rock 2.7 billion years ago.

Samarium (Sm), with an atomic weight of 147, decays to neodymium (Nd), with a weight of 143. But Nd has another isotope, 144Nd, which is not radioactive and does not change in concentration with time.

Dating the Stillwater Gabbro: For the Stillwater gabbro, the line has a slope indicating an age between 2.693 and 2.709 billion years.

In This Section

This 1.85-billion-year-old gneiss and the one-million-year-old granite vein cutting through it were dated by counting the number of fission tracks in the apatite (calcium phosphate) crystals contained in each rock.